1. Field of the Invention
This invention relates generally to electrochemical fuel cells and, more particularly, to an electrochemical fuel cell assembly with an electrode having an in-plane nonuniform structure.
2. Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) comprising a solid polymer electrolyte or ion exchange membrane disposed between two electrodes. The electrodes typically each comprise a substrate formed principally of a porous, electrically conductive sheet material, such as, for example, carbon fiber paper, carbon cloth or a composite material. The electrodes also comprise an electrocatalyst, disposed at the membrane/electrode substrate interfaces in the MEA, to induce the desired electrochemical reaction. The location of the electrocatalyst generally defines the electrochemically active area of the electrode or MEA.
Typically, the structure of the electrode and particularly the electrode substrate is substantially uniform, on a macroscopic scale, as it is traversed in-plane (that is, in the x- and y-directions, parallel to the planar major surfaces of the electrode substrate) at any depth.
In electrochemical fuel cells, the MEA is typically interposed between two substantially fluid impermeable separator plates (anode and cathode plates). The plates, which commonly have channels formed therein, act as current collectors, provide support to the MEA, provide means for access of the fuel and oxidant to the porous anode and cathode surfaces, respectively, and provide for the removal of product water formed during operation of the cells.
The conditions in an operating fuel cell vary significantly across the electrochemically active area of each electrode. For example, the water content of the each reactant streams varies as it moves in a reactant stream flow path across either electrode. In addition to the desired reactive component, the reactant stream may contain other components, such as carbon monoxide, which under certain conditions may be oxidized upon contact with certain electrocatalysts. Such oxidation will generally occur in a localized region in the inlet portion of the reactant flow path. Other conditions are more likely to occur in certain portions of the reactant flow path in a fuel cell, for example, reactant starvation, overheating, drying, flooding. Thus, the requirements and desired properties of the fuel cell electrode will be different in different regions.
Related U.S. Pat. No. 5,840,438 discloses the fuel cell performance benefits of imparting different fluid transport properties in a fuel cell electrode substrate, in a biased manner, between a reactant inlet and outlet. U.S. Pat. Nos. 4,851,377 and 5,702,839 disclose varying the electrocatalyst loading or composition, respectively, in a fuel cell electrode layer in a biased manner between a reactant inlet and outlet.
If the reactant flow path across the electrode is tortuous, it may be more difficult to provide the desired variation in electrode properties directly along the flow path. The reactant flow path may pass in and out of regions of the electrode in which the electrode properties have been modified to suit the conditions in the reactant stream.
It is particularly advantageous to incorporate an electrode having an in-plane nonuniform structure in a fuel cell in which the reactant travels in a substantially direct linear path across the electrode. In this configuration it is easier to control and attempt to optimize the variation in electrode properties along the reactant flow path. The variation in electrode properties may then be provided in a graded or banded manner as the electrode is traversed in-plane along such a substantially linear flow path.
If the reactant stream flow direction across the electrode is to be constant between an inlet and outlet, the variation is preferably provided in a biased manner along the path.
However, in a fuel cell in which the direction of flow of a reactant stream across an electrode is to be periodically reversed, it is desirable that the properties of the electrode vary in a substantially symmetrical manner, rather than in a biased manner, between the reactant inlet and outlet (which are periodically interchanging). This is preferred in order that the fuel cell performance is not significantly different for one reactant flow direction than the other.